About a dozen demonstrators had been arrested as of 3 p.m., and there were more than 20 demonstrators sitting inside and stop the building, apparently unwilling to move.

WATCH: OCCUPY D.C. STRUCTURE

It was a rare confrontation between police and the Washington demonstrators, who had up until now avoided the clashes that Occupy protests in other parts of the country have experienced, such as in Boston, New York and California.

The arrests Sunday were peaceful and orderly, but some demonstrators clung to the wooden framework of the structure and refused to move, including at least five who climbed onto the roof and perched on roof beams. Others sat inside the unfinished building and waited for police to enter and take them out. Some were handcuffed and put on the ground and later taken to waiting police vehicles.

Police appeared to be focused on the building itself and were not trying to remove the small tent city that has occupied the square for weeks. Numerous police cars — including a SWAT vehicle -- and emergency trucks surrounded the park, and authorities closed off 15th Street between I and K streets for much of the afternoon.

At 3:30 p.m., police began erecting metal barriers around the makeshift structure and assembled in a line along 15th Street and around the corner onto I Street. As some protesters rushed the gates, some officers pulled out pepper spray but did not use it. Protesters on top of the structure donned masks.

Shortly before 5 p.m., a building inspector who works for the National Park Service evaluated the structure and deemed it “dangerous,” said Anne Wilcox, a lawyer representing Occupy D.C. Police gave the occupiers three warnings to get out of the building before making arrests.

Following negotiations and warnings from a megaphone, police again moved into the structure at about 6 p.m., arresting at least eight people who had been sitting inside its base. Six people remained on the open roof, and police were working to remove them.

Occupy D.C. demonstrators believed police were just going to arrest those on the building, not those in the rest of the encampment.

“They are treating this as individual acts of disobedience,” Wilcox said.

The building, on the grassy southwest side of the square near the Metro entrance, sparked the issue early Sunday. Police surrounded it shortly after 10 a.m. and gave protesters an ultimatum: Take it down, or we will.

After discussing a way forward for an hour — demonstrators debated whether it was worth making a stand for the structure after lengthy good relations with police — roughly a dozen demonstrators remained inside at the deadline. Police told demonstrators they would need a permit to erect such a building.

Angelica Gatewood, 20, a student from the Pittsburgh School of Massage Therapy who joined the protest two days ago, said she did not feel as if demonstrators were intentionally trying to step up confrontations with police, as has occurred at other Occupy protests.

“I think they are trying to toe the line,” Gatewood said. “But I think a big part of the movement is challenging the status quo. It’s not going to be quiet.”

As police moved in, some chanted: “Leave us alone. These are the people who oppress us.”

Michael Patterson, 21, of Anchorage, said the structure was meant to symbolize the need to house the homeless. It had been covered with a blue tarp until shortly before the arrests.

“It is counterrevolutionary to occupy space with a permit,” Patterson told the crowd. “Why don’t the cops care about sheltering the homeless in the streets?”

Other demonstrators said the building was designed to provide demonstrators a place to go when it gets cold, and they were planning to build an “eco-friendly” heating device to make the structure sustainable.

Some protesters defended their decision to build the structure, calling it “temporary.”

“They said we were allowed to make structures as long as they were moved around every four days,” said Jennifer Ruse, 28, as she sat near the base of the statue of General McPherson. “This can be moved because we only put it up last night.”

Group chants escalated through the early afternoon, and there was an increasing amount of scuffling, shouting and shoving. Police used three horses and officers in riot helmets to create a barrier, and additional scuffles broke out as demonstrators tried to enter the structure. Some protesters were tackled and subdued.

Patterson later approached police and shouted in their faces, urging them to arrest him. And they did, dragging him away from the square as he shouted: “I didn’t serve in Iraq to have this happen to me.”

Despite the commotion, a group of Quakers had its weekly meeting in the south end of the park.

“I think it shouldn’t be happening,” said Frank Taylor, 62, of Arlington, who said the police response was not necessary. “The protesters are peaceful. They are not doing anything. This is an overreaction.”

But some tourists and D.C. residents drawn to the scene believed the protesters might be going too far.

“This is nuts. Instead of spending 50 days in the park, they could have been looking for jobs,” said Spence Levitas, 69, of Baltimore. “If they are not happy, move to Pakistan or Tahrir Square.”

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